In the operation of electronic equipment, such as data processors, it is often necessary to provide an indication of the state of various lines carrying electrical signals. For example, in applications involving an RS-232 type interface, test procedures may be implemented to obtain a visual indication of the state of each control and data line, including minimum operating voltage thresholds. Often, a large number of parallel interfaces must be monitored simultaneously, and the visual indication, discernible at a considerable distance from the monitor.
Present day monitoring circuits generally embody two separate amplifiers for signal voltage levels respectively above and below zero volts. Two separate voltage reference sources are required, along with a pair of output isolation elements, interposed between the respective output terminals of the amplifiers and the LED indicating device. The last mentioned device is driven by the monitor circuit, but its electrical characteristics do not play an active role in the monitor circuit design parameters.
It is apparent from the foregoing considerations that present day circuits used to monitor a large plurality of interfaces simultaneously, require a correspondingly large number of discrete components, along with LED indicators of comparatively large physical size. Power requirements for such an arrangement are also large. What is desirable is a monitor circuit with a minimal parts count and low power requirement. In fact, such a circuit is mandated in portable test equipment of the "suitcase" type, where space is extremely limited. The LED monitor circuit of the present invention fills such a need.